In recent years, providing small-cell base station apparatuses (hereafter referred to as HeNB: Home eNode B) each having a small cell radius in addition to macro-cell base station apparatuses (hereafter referred to as MeNB: Macro eNode B) each having a large cell radius has been considered so as to compensate a dead zone for mobile phones or to disperse data traffic. By providing HeNB, concentration of the traffic on a specific base station (hereafter referred to as eNB) is reduced, and a high user throughput as an entire network is expected.
However, in a time period when the number of network users is small such as midnight hours, a required throughput for each eNB declines, and thus constantly keeping a number of eNB in an active state at the same time would result in excessive energy consumption.
In order to solve this problem, a technique for suspending functions other than essential functions, based on the usage status of eNB, has been proposed. NPL 1 discloses a method for a shift to a power-saving mode for suspending the downlink transmission function of eNB when it is determined that no UE is present in an area covered by eNB.
FIGS. 1A to 1D illustrate a technique disclosed in NPL 1. In FIG. 1A, HeNB 102 is provided in a coverage of MeNB 101, and two mobile station apparatuses (hereafter referred to as UE: User Equipment) UE 103 and UE 104 are further present in the coverage of MeNB 101. In a case illustrated in FIG. 1A, no UE is present in the coverage of HeNB 102. Accordingly, HeNB under such a circumstance stops downlink transmission (FIG. 1B). Since MeNB 101 covers the area used to be covered by HeNB 102 (inside the broken line in FIG. 1B), the communication connection is secured even if UE is moved to the area.
In FIG. 1B, when new UE 105 and UE 106 enter the area covered by HeNB 102 (FIG. 1C) and HeNB 102 recognizes that UE is present in the area covered by HeNB 102 by detecting uplink signals from UE 105 or UE 106, for example, HeNB 102 starts downlink transmission (FIG. 1D).
Another technique disclosed in NPL 1 is illustrated in FIGS. 2A to 2D. FIG. 2A illustrates an example in which four HeNBs, that is, HeNBs 202 to 205 are provided in the coverage of MeNB 201. Furthermore, although UE 206 and UE 207 are present in the area covered by MeNB 201, the UEs establish connection with HeNB 203 and HeNB 204, respectively, instead of MeNB 201. In this case, since there is no UE that established connection with MeNB 201, MeNB under such a circumstance stops downlink transmission. In order to fill the dead zone due to the downlink transmission stopped by MeNB 201, HeNB 202 to HeNB 205 increase the downlink transmission power, expanding the coverage of each of the HeNBs (FIG. 2B).
In FIG. 2B, when new UE 208 to UE 216 enter the area covered by MeNB 201 (FIG. 2C), and MeNB 201 recognizes that the increase in the number of UEs in its own area, MeNB 201 starts the downlink transmission. HeNB 202 to HeNB 205 subsequently lower the downlink transmission power, setting the coverage of each HeNB back to a regular range (FIG. 2D).